There is a need for biomedical materials (preferably, biocompatible and biodegradable structural matrices) that facilitate tissue infiltration to repair/regenerate diseased or damaged tissue. Tissue engineering involves the development of a new generation of biomaterials capable of specific interactions with biological tissues to yield functional tissue equivalents. Most scaffolds can only introduce cells and/or signals after completion of the scaffold, due to the extreme conditions of the fabrication process, such as high or low temperature. The seeding of the cell in the inner part of a scaffold may be difficult, especially for larger objects with fine structural features. It would be very beneficial if cells could instead be introduced into the scaffold in situ. Furthermore, the addition of the chemical cues, such as growth factor, could be achieved in a controlled manner by fine tuning the degradation mechanism of biodegradable polymers such as collagen, polylactic acid and PCL.
It is known that nanotubes and nanofibers with core-sheath, hollow, or porous structures have many promising applications in a wide variety of technologies, including, for example, biomedical materials, scaffold and tissue regeneration and filtration. These fibers exhibit an especially advantageous combination of properties of being light-weight, flexible, permeable, strong and resilient in linear, two-dimensional and three-dimensional structures. In terms of biomedical application, there is great interest in devising a scaffold structure that mimics the tissue for better tissue regeneration. Highly aligned structure can be best represented by the structure of the nerve, vascular and some other tissues, or their parts.
Electrospinning is a process that relies on electric charges to deform a conical droplet of polymeric solution ejected from a nozzle tip into ultra-fine fibers. Electrospinning makes it relatively easy to spin continuous nanofibers from many different materials, including, but not limited to, polymers. Electrospinning provides a straightforward and practical way to produce fibers with diameters ranging from a few to about two-thousand nanometers. Electrospinning represents a versatile, low-cost method for producing micron- to nano-scale fibers in the form of either membrane or 3-D structure. An apparatus for preparing electrospun nanofibers is introduced in Polym Int 56:1361-1366, 2007. WO 2005095684 is directed to substantially continuous fibers which have a core-and-shell structure; however, these fibers are randomly arranged, not aligned and packed. Currently there are only limited reports of production of highly aligned electrospun fibers, either by collecting fibers with a rotating disc (A. Theron, E. Zussman and A. L. Yarin, “Electrostatic field-assisted alignment of electrospun nanofibres,” Nanotechnology, Vol. 12, P 384-390, 2001), drum (P. Katta, M. Alessandro, R. D. Ramsier, and G. G. Chase, “Continuous Electrospinning of Aligned Polymer Nanofibers onto a Wire Drum Collector,” Nano Lett., Vol. 4, No. 11, 2004) or frame (H. Fong, W-D. Liu, C-S. Wang, R A. Vaia, “Generation of electrospun fibers of nylon 6 and nylon 6-montmorillonite nanocomposite,” Polymer, 43(3), P 775-780, 2002), or with a set of parallel conductive substrates (Dan Li, Yuliang Wang, and Younan Xia, “Electrospinning of Polymeric and Ceramic Nanofibers as Uniaxially Aligned Arrays,” Nano Lett., Vol. 3, No. 8, 2003). Some degree of fiber orientation with the aid of the multiple-field technique has also been reported (J. M. Deitzel, J. Kleinmeyer, J. K. Hirvonen, Beck TNC., “Controlled deposition of electrospun poly(ethylene oxide) fibers,” Polymer, Vol. 42, P 8163-8170, 2001). Moreover, U.S. Pat. No. 7,575,707 discloses a method for electrospinning nanofibers having a core-sheath, tubular, or composite structure.
However, the above references all have the disadvantage of limited alignment, which becomes even worse as the deposited fiber layers grow thicker. Also troublesome is the very limited production speed and/or small production quantity of electrospinning with needle type spinnerets, which makes it lack industrial value.
There are few reports on the preparation of nano/micro tube by electrospinning process. Li et al reported preparing nanotube via a single capilliary electrospinning (Li, X. H. S., Chang L. and Liu, Yi C., A Simple Method for Controllable Preparation of Polymer Nanotubes via a Single Capillary Electrospinning. Langmuir, 2007, 23: p. 10920-10923). Core/sheath, PPy/PVP and hollow PVP nanofibers were also prepared by Srivastava using hydrodynamic fluid focusing microchannel design (Srivastava, Y. R., C.; M.; Thorsen, T., Electrospinning hollow and core/sheath nanofibers using hydrodynamic fluid focusing; Microfluid Nanofluid, 2005, 5:p. 455-458). Di et al. prepared zeolite hollow fiber by calcinations of the as-spun fibers from coaxial electrospinning of the silicalite-1 nanoparticles in poly(vinyl pyrrolidone) (PVP)/ethanol solution to the outer shell and paraffin oil acted as the inner liquid (Di, J. C. C., H. Y., Wang, X. F., Zhao, Y, Yu, J. H. and Xu, R. R., Fabrication of Zeolite Hollow Fibers by Coaxial Electrospinning. Chem. Mater., 2008, 20(11): p. 3543-3545). However, none of them was able to prepare highly aligned and highly packed micro-tube. Accordingly, there is a need to develop structurally aligned and closely packed fibers.